Importantly for odorant discrimination, each olfactory neuron expresses only one of the two alleles of only one of the thousand possible olfactory receptors. The basic question of how each neuron chooses to express only one olfactory receptor remains a mystery, however. I propose to investigate the epigenetic differences between olfactory receptor alleles that may distinguish alleles prior to choice, or may result from the act of choosing. Specifically, cell lines will be generated in which the replication timing characteristics of olfactory receptor genes can be determined. These cell lines will be used to search for important epigenetic modifications, such as nuclease sensitivity, DNA methylation and histone acetylation that may differ between earlier and late replicating alleles of olfactory receptors. A correlation between replication timing and epigenetic marking will be sought. This information will be used to guide the characterization of the epigenetic differences between alleles of olfactory receptor clusters in populations of isolated olfactory neurons, all of which express a specific olfactory receptor allele. In addition to addressing the critical question of odorant discrimination, these experiments will provide useful insight into the regulation of other asynchronously replicating gene families.